As communication networks have expanded and diversified to meet the needs of a variety of subscriber/users, the continued successful operation of relay satellites, which constitute a critical component of each network link, has continued to draw increasing attention, especially with respect to the matter of power allocation among the links. In order for any relay link to operate successfully over a variety of link conditions, i.e. to accommodate changing levels of signal attenuation caused, for example, by rainfall between the relay satellite and the earth station, the satellite TWT power allocated to each downlink terminal is tailored to provide a prescribed degree of excess power (rain fade margin). Typically, this power differential may be on the order of 6 dB higher than that required to close the link in clear weather. Because heavy rainfall is infrequent and all terminals do not experience rainfall simultaneously, this rain fade margin is wasted most of the time.
In an effort to circumvent this highly inefficient allocation of effectively unused excess power, adaptive link power control (ALPC) schemes such as described in U.S. Pat. Nos. 4,261,054 to Scharla-Nielsen and 4,228,538 to Scharla-Nielsen et al and assigned to the Assignee of the present application, have been proposed. In an ALPC satellite network, each link monitors its received signal quality and sends appropriate power control commands to the transmitter terminal in an attempt to maintain desired link performance during rain fades. In response to these power control commands, the uplink transmitter causes power to be drawn from a common or shared power pool that is normally held in reserve in the satellite TWT until it is actually required by individual downlinks. Because of the statistical nature of the occurrence and intensity of rain fades, the size of this power pool or rain margin in the TWT is considerably less than the above-referenced 6 dB figure, so that the ALPC satellite network can support more terminals and/or higher data throughput. Now although the ALPC approach offers a reduction in wasted power and thereby an improvement in throughput capacity, it subjects the network to a potential "lockup" condition.
More particularly, should the power in a satellite channel (which may be shared by a number of users) increase to the non-linear portion of its TWT power transfer characteristic, due to the pervasive action of ALPC links responding to downlink fades, the intermodulation products (intermods) between signals in the satellite TWT generate an effective increase in the apparent noise level in the channel. To offset this increase in noise level, the ALPC mechanism increases the power transmitted by the uplink transmitter, resulting in increased satellite TWT power which, in turn, further increases the intermod noise seen by each link. Along the TWT power transfer characteristic there is a threshold (or "avalanche point") beyond which an increase in signal power cannot compensate for the increase in intermod noise produced by the signal and the ALPC mechanism will drive the channel into saturation. Once the channel has been driven into saturation, ALPC will hold it there forever if no external escape mechanism is provided to undo this "lockup" condition (the channel is "locked-up" in saturation by ALPC).
Not only do all users of the locked-up channel suffer poor performance for the duration of this condition, but once avalanche is reached, it is normally only a matter of seconds before the entire system goes into lockup due to the influence of ALPC. Moreover, during the lockup condition, the deterioration of performance in the control link can be severe enough to make recovery very difficult and time consuming (especially if the control link is also involved in the fade that caused the lockup condition).